Try  each of the following steps until resolved:
  1. Ensure the same version of Word is being used to create and edit letters. For example, if one user uses Word 2003 to create the letter but another opens the output file in Word 2007, the different versions can affect the document's appearance.
  2. Ensure that margins of all merge documents in Merge Wizard are the same. Use margin settings from Format menu in Word. Tabs and returns should be used minimally.